But marathons and other endurance races could cause structural changes to the heart, leading to heart attacks.

Dr James O'Keefe, of St Luke's Hospital in Kansas city, who reviewed previous research, said: "Physical exercise, though not a drug, possesses many traits of a powerful pharmacological agent.

"Daily physical activity can be highly effective for prevention and treatment of many diseases, including coronary heart disease, hypertension, heart failure and obesity.

"However, as with any pharmacological agent, a safe upper dose limit potentially esists, beyond which the adverse effects of physical exercise may outweigh its benefits."

COMMENT RULES: Comments that are judged to be defamatory, abusive or in bad taste are not acceptable and contributors who consistently fall below certain criteria will be permanently blacklisted. The moderator will not enter into debate with individual contributors and the moderator’s decision is final. It is Belfast Telegraph policy to close comments on court cases, tribunals and active legal investigations. We may also close comments on articles which are being targeted for abuse. Problems with commenting? customercare@belfasttelegraph.co.uk